Hi!
well i use JapBe [Just another PureBasic editor] made by GPI. It has auto-complete too and more. PureBasic 4.0 will have a new editor, but until now try japbe. I am not sure that it works with demo, but i see no reason why it shouldnt..
http://gpihome.de/purebasic/jaPBe/
It comes with a lot of styles and you can even configure it yourself.
Try it out, Its free!
edit: Since its a beta, install then update with the InterNetUpdater.rar found on the same page! you just run the exe in the rar file then it will update.
Be aware of the "Aqua" theme as it freezes on my computer
but try some others, or make one yourself.